This invention relates to electrochromic devices. In one aspect it relates to electrically controllable display devices. In another aspect it relates to electrically tunable optical or light filters. In yet another aspect it relates to a chemical sensor device which employs a color changing film.
There are many uses for electrically controllable display devices. A number of such devices have been in commercial use for some time. These display devices include liquid crystal displays, light emitting diode displays, plasma displays, and the like. Light emitting diode and plasma display panels both suffer from the fact that they are active, light emissive devices which require substantial power for their operation. In addition, it is difficult to fabricate light emitting diode displays in a manner which renders them easily distinguishable under bright ambient illumination. Liquid crystal displays suffer from the disadvantage that they are operative only over a limited temperature range and have substantially no memory within the liquid crystal material. Further, the visibility of many liquid crystal displays decreases as the viewer moves a few degrees off axis.
Electrochromic displays have been developed which display information through a change in color of portions in the display. In certain of these displays the color change is accomplished by way of reversible electro-precipitation of certain cations onto a transparent electrode. In certain other of these displays a metal ion in the electrolyte is reversibly reacted with a transparent electrode. In these known electrochromic displays, coloration is induced employing an external potential. By reversing the original potential, or by applying a new potential, it is possible to cancel, erase or bleach the visible coloration. These steps of color induction and erasure are defined as cycling.
Because of their operative mechanisms, the known electrochromic display devices have suffered the drawbacks of requiring substantial power and/or time to write or erase displayed information.
Rare earth diphthalocyanines are known to have electrochromic properties in which the color of the diphthalocyanine can change over a period of about 8 seconds upon application of a potential difference across an electrochemical cell having a diphthalocyanine film on one of the electrodes. The diphthalocyanine does not require large amounts of power to change color, but the long period required for the color to change makes known diphthalocyanine performance characteristics unacceptable when measured against display requirements.
Nicholson, U.S. Pat. No. 4,184,751 describes electrochromic display devices using a metal diphthalocyanine complex as the electrochromic material in which displayed information can be switched in 200 milliseconds or less by constructing the device so that the apparent RC time constant of the overall structure is one second or less. A multi-color, i.e., more than two color, display is achieved through use of a range of voltages applied between display and counter electrodes. Color reversal of displayed information and the background against which it is displayed is achieved through use of display electrodes in the background portions of the viewing area as well as in the character segments.
In a simpler tYpe of displaY device where color reversal is not required the background portions of the viewing area are often provided with deposits of the display material surrounding and conforming to the outlines of the segmented character electrodes. This feature is intended to provide a uniform appearance to obscure the character electrodes when the display device is in the erase condition.
In such a device, the metal diphthalocyanine display material has an initial color in both the background regions and on the character or display electrodes. Upon electrical cycling, however, the display material on the display electrodes typically does not return to the precise initial color. This is objectionable in a display device.
Several approaches have been proposed for use in solving the problem of the failure of the erased display electrodes to match the color of the background. Most such proposals are relatively expensive either in terms of materials or processing steps. Nicholson et al., in U.S. patent application Ser. No. 330,041, filed Dec. 11, 1981, now abandoned, disclose a simple, inexpensive method for treating an electrochromic display material in order that the background and cycled materials have substantially the same color. The method comprises treating a deposited display material with a liquid vehicle containing substituted imidazoline and silicone glycol surfactants, then driving the liquid vehicle from the deposited display material. Where the display material is lutetium diphthalocyanine, the resulting color is olive-green. In the written state, the color is bright green; in the erased state the color is olive-green.
Matrix display devices contain one or more arrays of many small elements or dots of color changing material that can be selectively activated or switched to form virtually any alphanumeric or graphic pattern. To create such patterns and erase them at will some means must be provided to access each element independently without activating those in the surrounding area. It has been proposed to build an integrated drive matrix of thin-film transistors into a display device so that each element is provided, in effect, with a separate switch connecting it to the power supply. A simpler approach is to use a multiplexed addressing scheme. Nicholson U.S. patent application Ser. No. 327,856, filed Dec. 7, 1981, now U.S. Pat. No. 4,456,337, describes a chemically coupled display device which may be addressed by direct multiplexing of two sets of parallel conductive, linearly-extending electrodes disposed at right angles.
Electrically tunable filters consist of a film of color-changing filter material wherein the color of the filter is electrically tunable. Nicholson, U.S. patent application Ser. No. 451,294 filed Dec. 20, 1982, now U.S. Pat. No. 4,501,472 describes an electronically tunable filter and an electronically switchable light valve which is capable of being reversibly changed from light transmissive in one state to opaque in another.
It is an object of the present invention to provide an improved chemically coupled color-changing display device.
It is another object of this invention to provide an improved tunable electrochromic filter.
It is yet another object of this inventlon to provide an improved non-electronic chemical sensing device.
Other objects, aspects, and advantages of the present invention will be apparent to those skilled in the art.